This invention relates generally to apparatus for generating defibrillation waveforms, and more particularly to a solid-state circuit for generating a multiphasic defibrillation waveform in an external defibrillator.
One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient""s chest. The switching mechanism used in most contemporary external defibrillators is a mechanical high-energy transfer relay. A discharge control signal causes the mechanical relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.
The American Heart Association has recommended a range of energy levels for the first three defibrillation pulses applied by an external defibrillator. The recommended energy levels are: 200 joules for a first defibrillation pulse; 200 or 300 joules for a second defibrillation pulse; and 360 joules for a third defibrillation pulse, all within a recommended variance range of no more than plus or minus 15 percent according to standards promulgated by the Association for the Advancement of Medical Instrumentation (AAMI). These high-energy defibrillation pulses are required to ensure that a sufficient amount of the defibrillation pulse energy reaches the heart of the patient, after accounting for energy dissipated in the chest wall of the patient.
The mechanical relay used in contemporary external defibrillators has traditionally allowed a monophasic waveform to be applied to the patient. It has recently been discovered, however, that there may be certain advantages to applying a biphasic, rather than a monophasic, waveform to the patient. For example, preliminary research indicates that a biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse.
One prior art circuit for generating a biphasic waveform of the energy levels recommended by the American Heart Association is illustrated in U.S. Pat. No. 5,824,017, which is hereby incorporated by reference. FIG. 1 of U.S. Pat. No. 5,824,017 has been reproduced as FIG. 1 herein. The circuit of FIG. 1 shows a defibrillator 8 which includes a mechanical relay 35. As will be described in more detail below, in order to make the defibrillator 8 into an entirely solid-state defibrillator, the mechanical relay 35 would have to be replaced with a solid-state relay or else eliminated. However, certain problems such as leakage currents would be associated with an entirely solid-state defibrillator. In order to provide a better understanding of the problems associated with an entirely solid-state defibrillator, the structure and operation of the circuit of FIG. 1 will now be described in detail.
FIG. 1 includes a block diagram of an external defibrillator 8 that is connected to a patient 16. The defibrillator includes a microprocessor 20 that is connected to an energy storage capacitor 24 via a charging circuit 18. During the operation of the defibrillator 8, the microprocessor 20 controls the charging circuit 18 by a signal on a control line 25 to charge the energy storage capacitor 24 to a desired voltage level. In order to generate the necessary defibrillation pulse for external application to a patient, the energy storage capacitor 24 is charged to between 100 volts and 2,200 volts. To monitor the charging process, the microprocessor 20 is connected to a scaling circuit 22 by a pair of measurement lines 47 and 48, and by a control line 49. The scaling circuit 22 is connected to the energy storage capacitor 24 by a bridge line 28, which connects to the negative lead of the capacitor 24, and by a line 30, which connects to the positive lead of the capacitor 24. A clock 21 is also connected to the microprocessor 20.
After charging to a desired level, the energy stored in the energy storage capacitor 24 may be delivered to the patient 16 in the form of a defibrillation pulse. An output circuit 14 is provided to allow e controlled transfer of energy from the energy storage capacitor 24 to the patient 16. The output circuit 14 includes four switches 31, 32, 33, and 34, each switch on a leg of the output circuit 14 arrayed in the form of an xe2x80x9cHxe2x80x9d (hereinafter the xe2x80x9cH-bridgexe2x80x9d output circuit). Switches 31 and 33 are coupled through a protective component 27 to the positive lead of the energy storage capacitor 24 by a bridge line 26. The protective component 27 has both inductive and resistive properties, and thereby limits the current and voltage changes from the energy storage capacitor 24.
Switches 32 and 34 are coupled to the energy storage capacitor 24 by a bridge line 28. The patient 16 is connected to the left side of the H-bridge by an apex line 17, and to the right side of the H-bridge by a sternum line 19. As depicted in FIG. 1, the apex line 17 and the sternum line 19 are connected to electrodes 15A and 15B, respectively, by a patient isolation relay 35. The microprocessor 20 is connected to the switches 31, 32, 33, and 34 by control lines 42A, 42B, 42C, and 42D, respectively, and to the patient isolation relay 35 by control line 36. Application of appropriate control signals by the microprocessor 20 over the control lines causes the switches of the output circuit 14 to be appropriately opened and closed (described in more detail below), whereby the output circuit 14 conducts energy from the energy storage capacitor 24 to the patient 16.
In order to conduct a first phase of a biphasic pulse from the energy storage capacitor 24 to the patient 16, switches 31 and 32 are closed along with relay 35. Thus, during the first phase, energy travels from the positive terminal of the capacitor 24 down through switch 31, out lines 17 and 15A to the patient 16, and then back from the patient 16 through lines 15B and 19, down through switch 32 to the negative terminal of the capacitor 24. The first phase is ended by opening switches 31 and 32 before the capacitor 24 is completely discharged. Then the second phase of the biphasic defibrillation pulse is begun by closing switches 33 and 34 with relay 35 also closed. Thus, during the second phase, energy travels from the positive terminal of the storage capacitor 24 down through switch 33, out lines 19 and 15B to the patient 16, and then back from the patient 16 through lines 15A and 17, and down through switch 34 to the negative terminal of the capacitor 24. It can be seen with reference to FIG. 1 that the travel of energy through the patient 16 during the first phase of the biphasic defibrillation pulse is opposite in direction to the travel of energy through the patient 16 during the second phase of the biphasic defibrillation pulse.
The mechanical relay 35 is a large, expensive, and relatively finicky component. It would be desirable to eliminate the mechanical relay if possible and replace it with solid-state switches, or else eliminate it altogether. However, there are at least two problems with either of these solutions. The first problem has to do with leakage currents, and the second problem has to do with the shorting of a defibrillation pulse from a simultaneously connected second defibrillator.
Leakage currents are relatively small currents that flow through solid-state devices even when they are supposed to be in the off state. For example, solid-state devices such as SCRs and IGBTs in some applications may have a leakage current of around 1 milliamp. In a mechanical relay, no leakage currents occur because the mechanical contacts and blades of the mechanical relay are physically separated from one another when the relay is open, thus preventing any current from flowing. In contrast, in solid-state devices, leakage currents can occur because solid-state devices by definition have no moving parts that can be physically separated. Instead, solid-state devices typically rely on gate voltages or similar phenomena to control the current flow. Even with the gate voltages all the way off, a small amount of leakage current usually still results through the semiconductor elements.
With reference to FIG. 1, if the relay 35 was made to be solid-state, one path in the output circuit 14 through which the leakage currents would reach the patient 16 is from the positive terminal of the capacitor 24, through solid-state switch 31, down through line 17, through a solid-state switch at relay 35, through line 15A, through the patient 16 and back through line 15B, through another solid-state switch at relay 35 to line 19, down through switch 32 and back via line 28 to the negative terminal of the capacitor 24. With leakage currents of about 1 milliamp, this creates a leakage current through the patient of about 1 milliamp, which is far greater than the acceptable 100 microamp current in an external defibrillator for the patient. The acceptable leakage current in other circumstances may be even less (e.g., the acceptable leakage current for direct defibrillation during surgery may be 10 microamps or less).
Another problem with substituting solid-state switches for the mechanical relay is the short circuiting of a defibrillation pulse from a simultaneously attached second defibrillator. In other words, the situation may sometimes occur where once a first defibrillator is connected to a patient in an emergency situation by a first emergency response team, a second defibrillator may be connected to the patient at a later time by a second emergency response team while the first defibrillator is still attached. In such a circumstance, the switches of the first defibrillator circuitry must be able to withstand a shock from the second defibrillator, without breaking down and allowing the defibrillation shock from the second defibrillator to short circuit through the first defibrillator circuitry rather than being applied to the patient.
More specifically, as shown in FIG. 1, if a second defibrillator were hooked across the patient 16 such that its electrodes were coupled to the same general areas of the patient as the electrodes 15A and 15B, a defibrillation pulse from the second defibrillator could short circuit through the relay 35 to line 17, down switch 34, up switch 32 to line 19 such that the energy would travel through this path rather than through the patient 16. Alternatively, the energy could short circuit through the relay 35 through line 17 up through switch 31, down through switch 33 and out through line 19 and out through relay 35, rather than traveling through the patient 16. This is only a problem when solid-state switches are used that often do not have high enough voltage tolerances to withstand a shock from another external defibrillator. This was not generally a concern with mechanical relays because the physical separation of the relay components helped prevent voltage breakdown across the relay.
The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to a solid-state defibrillation circuit that limits undesired currents through the defibrillator.
A solid-state defibrillation circuit that allows a multiphasic defibrillation pulse to be discharged to a patient from an energy storage device, preferably an energy storage capacitor, is disclosed. The defibrillation circuit applies the defibrillation pulse to the patient through an output circuit and electrodes when the electrodes are coupled to the patient. The defibrillation circuit also includes a current limiting circuit that limits the leakage currents that flow to the patient and that also prevents short circuiting of currents from a second defibrillator.
In accordance with one aspect of the invention, the current limiting circuit comprises a plurality of resistors coupled to the solid-state output circuit. In an embodiment where the output circuit is in the form of an H-bridge, the current limiting circuit includes a resistor coupled in parallel with each leg of the H-bridge. Leakage currents from the solid-state relay switches will be distributed through the resistors, causing the leakage currents flowing through the output circuit to become more balanced across the H-bridge. This current distribution reduces the voltage differential between the circuit nodes to which the electrodes are coupled. In this manner, leakage currents to the patient are reduced. The leakage currents are preferably reduced to a level of 100 microamps or less.
In accordance with another aspect of the invention, the current limiting circuit includes a shunt resistor coupled between the circuit nodes that are coupled to the electrodes. The shunt resistor is designed to shunt leakage currents away from the patient. In one embodiment, the shunt resistor has a value of about 1 Kohm.
In accordance with another aspect of the invention, the current limiting circuit includes a non-linear element, such as an MOV or a tranzorb, in series with the defibrillator output. These elements limit leakage currents by conducting very little current below a threshold voltage. Normal defibrillator operation is allowed in that the elements conduct normal current when high voltages are present.
In accordance with yet another aspect of the invention, the current limiting circuit includes a diode coupled in series with one of the switches of the output circuit. The function of the diode is to prevent voltage breakdown when a high-energy shock is applied from an external source, such as from a second defibrillator.
It will be appreciated that the disclosed solid-state multiphasic defibrillation circuit is advantageous in that it eliminates the need for a mechanical relay that can be large, expensive, and difficult to control.